Trapped-Ion Quantum Processor Advances with Two-Qubit Register Implementation

The article explores the implementation of arbitrary circuits on a universal two-qubit register in a trapped-ion quantum computer. The researchers used a two-ion Coulomb crystal of 9Be ions and chip-integrated microwave addressing to implement a universal set of quantum gates.

They also used microwave micromotion sideband transitions for individual-ion addressing. The research is significant for the development of quantum computing, particularly trapped-ion quantum processors. The successful implementation of arbitrary circuits on a universal two-qubit register is a significant advancement in the field.

What is the Significance of Arbitrary Quantum Circuits in a Trapped-Ion Quantum Processor?

The article discusses the implementation of arbitrary circuits on a universal two-qubit register that can function as the computational module in a trapped-ion quantum computer. This is based on the quantum charge-coupled device architecture. A universal set of quantum gates is implemented on a two-ion Coulomb crystal of 9Be ions using only chip-integrated microwave addressing. Individual-ion addressing is implemented using microwave micromotion sideband transitions. The researchers have obtained upper limits on addressing crosstalk in the register. Arbitrary two-qubit operations are characterized using the cycle benchmarking protocol.

Trapped ions are considered promising platforms for building a universal, general-purpose digital quantum computer. Key features include the availability of long-lived hyperfine state pairs, long coherence times, high-fidelity quantum gates, and all-to-all connectivity. Microfabricated and surface-electrode ion traps provide a scalable platform for the implementation of the quantum charge-coupled device architecture.

An elementary quantum core could consist of a two-qubit computation register combined with a junction and suitable storage registers, as well as an individual ion readout and state preparation register. In terms of scaling the trapped-ion platform, it is desirable to integrate core aspects of the control of trapped ion qubits into a scalable microfabricated trap structure.

How is the Two-Qubit Register Implemented?

The two-qubit register is implemented in a single-layer microfabricated Paul trap. The qubits are encoded in the internal hyperfine ground states of two 9Be ions on a first-order magnetic-field independent transition. The register is designed for trapping ions at an approximate height of 70µm from the surface. A total of 10 DC control electrodes provide axial confinement and enable fine adjustment of the position and orientation of a two-ion crystal, while one split RF electrode provides the radial confinement.

A microwave signal applied to an integrated S-shaped microwave conductor will produce a two-dimensional quadrupole microwave field in the radial plane. The center of the quadrupole field coincides with the pseudopotential minimum of the Paul trap. The resulting trap frequencies are ωxyz = 2π 110,162,016,251 MHz. The ion crystal’s radial lower-frequency in-phase and out-of-phase motional modes are ground-state cooled to nIP = 0.42, nOOP = 0.12, while the other modes of motion are Doppler cooled only.

What is the Role of Micromotion Sidebands in Individual-Ion Addressing?

The RF field oscillates at ωRF=2π882MHz. If an ion is held at a position of non-vanishing RF field, this will result in micromotion at ωRF with an amplitude rMM. If the microwave conductor is driven at ω0=ωRF, where ℏω0=EE, then in the comoving frame of the ion, it will experience an oscillating field component at ω0. As a result, the ion will undergo Rabi oscillations micromotion sidebands with a Rabi rate.

Individual-ion addressing can be implemented if one ion experiences strong micromotion while the other ion ideally experiences vanishing micromotion. Two different configurations of the crystal can be prepared to address each one of the ions. This is accomplished by finding two sets of DC potentials, each one twisting the crystal into the appropriate configuration. In each configuration, the ion to be hidden from the interactions sits as close as possible to the null of the radially confining RF field.

How is the Calibration Procedure Conducted?

Transport to and from each configuration is done adiabatically in approximately 100µs. Fine adjustment of the crystal’s final position is done via a superimposed set of DC potentials, which are designed to displace the crystal in the radial plane. The calibration procedure is based on minimizing the Rabi frequency of the qubit encoded in the non-addressed ion and is accomplished by changing the values of the DC potentials while performing a Rabi flopping experiment.

What are the Implications of this Research?

The research demonstrates the execution of arbitrary two-qubit circuits on a universal two-qubit computation register. All quantum gates were executed using on-chip integrated elements only. The researchers placed upper bounds on crosstalk infidelities in individual-ion addressing and demonstrated how to establish the proper phase relationship for combining individual-ion addressing with two-qubit gates in their approach. Finally, they showed the execution of arbitrary two-qubit circuits by applying the cycle benchmarking protocol.

This research is significant as it contributes to the development of quantum computing, particularly in the area of trapped-ion quantum processors. The successful implementation of arbitrary circuits on a universal two-qubit register represents a significant step forward in the field. The findings of this research could have far-reaching implications for the future of quantum computing.